Percutaneous transluminal coronary angioplasty (PTCA) is used to reduce arterial build-up of cholesterol fats or atherosclerotic plaque. Typically a first guidewire of about 0.038 inches in diameter is steered through the vascular system to the site of therapy. A guiding catheter, for example, can then be advanced over the first guidewire. The first guidewire is then removed. A balloon catheter on a smaller 0.014 inch diameter guidewire is advanced within the guiding catheter to a point just proximal to the stenosis. The second guidewire is advanced into the stenosis, followed by the balloon at the distal end of the catheter. The balloon is then inflated causing the site of the stenosis to compress into the arterial wall. The dilatation of the occlusion, however, can form flaps, fissures and dissections which threaten re-closure of the dilated vessel or even perforations in the vessel wall. Implantation of a metal stent can provide support for such flaps and dissections and thereby prevent reclosure of the vessel or provide a patch repair for a perforated vessel wall until corrective surgery can be performed. Reducing the possibility of restenosis after angioplasty reduces the likelihood that a secondary angioplasty procedure or a surgical bypass operation will be necessary.
An implanted prosthesis such as a stent can preclude additional procedures and maintain vascular patency by mechanically supporting dilated vessels to prevent vessel collapse. Stents can also be used to repair aneurysms, to support artificial vessels as liners of vessels or to repair dissections. Stents are suited to the treatment of any body lumen, including the vas deferens, ducts of the gallbladder, prostate gland, trachea, bronchus and liver. The body lumens range in size from the small coronary vessels to the 28 mm aortic vessel. The invention applies to acute and chronic closure or reclosure of body lumens.
A typical stent is a cylindrically shaped wire formed device intended to act as a permanent prosthesis. A stent is deployed in a body lumen from a radially compressed configuration into a radially expanded configuration which allows it to contact and support a body lumen. The stent can be made to be radially self-expanding or expandable by the use of an expansion device. The self expanding stent is made from a resilient springy material while the device expandable stent is made from a material which is plastically deformable. A plastically deformable stent can be implanted during a single angioplasty procedure by using a balloon catheter bearing a stent which has been crimped onto the balloon. Stents radially expand as the balloon is inflated, forcing the stent into contact with the body lumen thereby forming a supporting relationship with the vessel walls.
The biocompatable metal stent props open blocked coronary arteries, keeping them from reclosing after balloon angioplasty. A balloon of appropriate size and pressure is first used to open the lesion. The process is repeated with a stent crimped on a balloon. The stent is deployed when the balloon is inflated. The stent remains as a permanent scaffold after the balloon is withdrawn.
U.S. Pat. No. 4,886,062 to Wiktor for "Intravascular Radially Expandable Stent and Method of Implant" discloses a two-dimensional zig-zag form, typically a sinusoidal form.
U.S. Pat. No. 5,409,495 to Osborn for "Apparatus for Uniformly Implanting a Stent" discloses elastic restraining bands which exert a force at the proximal and distal ends of the balloon equal and opposite to that generated by the combined resistance of the sleeve and the stent tending to deform the balloon. In this way, the uneven expansion (end effects) are limited when the balloon is expanded which, in turn, inhibits a "dog boning" deformation at the proximal and distal regions of the balloon. FIGS. 3-6 show a balloon of complex manufacture.
As stent metal mass increases in stents having elements that can expand independently in the longitudinal direction, there is a tendency towards longitudinal compression at the center of the stent when expanded. The increased metal mass creates more radial hoop strength which in turn increases the mount of force required to expand the stent. The center of the stent has more radial hoop strength than the ends of the stent. As a result, the balloon expands first at the distal and proximal ends before expanding at the center. This creates a dumbbell shaped balloon. With the stent ends expanding first, the stent slides down the expanded balloon ends toward the center of the balloon which is as yet unexpanded because of the stent's greater radial hoop strength in the center. When the balloon ends have expanded completely, the stent has been compressed to approximately one-half of its original crimped length. Because the stent is compressed toward the center of the balloon, complete balloon expansion may not be possible. What is needed is a method of stent deployment which results in uniform stent expansion.